This invention relates to a parallel-axis differential having side gears and planetary gears in parallel with the side gears.
As disclosed in U.S. Pat. No. 5,194,054, the parallel-axis differential for vehicles includes a housing rotationally driven by an engine, a pair of side gears rotatably received in and coaxial with the housing, and at least one pair (usually, three or four pairs) of planetary gears rotatably supported within the housing and being in parallel relation with the side gears. The paired planetary gears are in engagement with the pair of side gears, respectively, and also in engagement with each other. The pair of side gears are connected to end portions of a pair of coaxial output shafts, respectively.
In the differential thus constructed, pockets extending in parallel with the axis of the housing are formed in an inner periphery of the housing. The planetary gears are received in the pockets, respectively. For differential rotation, the planetary gears are rotated with outer peripheral surfaces (top faces of teeth) being in contact with inner peripheral surfaces of the pockets, respectively. At that time, since each planetary gear receives a perpendicular force to its axis from the side gears and the other planetary gear, each planetary gear is rotated while fractionally contacting the inner peripheral surface of the pocket. Owing to this friction, torque transmitting from the housing to the low-speed rotating side gears becomes larger than that of the high-speed rotating side gears.
In the above-mentioned differential, since the perpendicular force to the axis is not uniformly applied to each planetary gear over its entire length, each planetary gear is slightly inclined to the extent of minor play. As a consequence, edges of teeth located on one end of each planetary gear are brought into contact with the inner peripheral surface of the pocket in a very small contact area. The edges of the teeth are formed on the boundary area between top faces of the teeth and end faces of the teeth. Since each planetary gear is rotated relative to the housing in that contacting state, the teeth of the planetary gear and the inner peripheral surface are worn.